In a turbomachine, such as a gas turbine engine, air is pressurized in a compressor then mixed with fuel and burned in a combustor to generate hot combustion gases. The hot combustion gases are expanded within a turbine of the engine where energy is extracted to power the compressor and to provide output power used to produce electricity. The hot combustion gases travel through a series of turbine stages. A turbine stage may include a row of stationary airfoils, i.e., vanes, followed by a row of rotating airfoils, i.e., turbine blades, where the turbine blades extract energy from the hot combustion gases for powering the compressor and providing output power.
Since the airfoils, i.e., vanes and turbine blades, are directly exposed to the hot combustion gases as the gases pass through the turbine, these airfoils are typically provided with internal cooling circuits that channel a coolant, such as compressor bleed air, through the airfoil and through various film cooling holes around the surface thereof. For example, film cooling holes are typically provided in the walls of the airfoils for channeling the cooling air through the walls for discharging the air to the outside of the airfoil to form a film cooling layer of air, which protects the airfoil from the hot combustion gases.
Film cooling effectiveness is related to the concentration of film cooling fluid at the surface being cooled. In general, the greater the cooling effectiveness, the more efficiently the surface can be cooled. A decrease in cooling effectiveness causes greater amounts of cooling air to be employed to maintain a certain cooling capacity, which may cause a decrease in engine efficiency.